The present invention relates to a liquid crystal display apparatus, and, more particularly, to an active matrix liquid crystal display apparatus.
Various examples of liquid crystal display apparatus are disclosed, for example, in Japanese Patent Publication No. 63-21907 (1988), UP, W091/10936 and Japanese Patent Application Laid-Open No. 6-222397 (1994), in which a pair of comb electrodes are used to apply an electric field to a liquid crystal in a direction parallel to the surface of a substrate. However, in a display system of this type wherein the direction of an electric field applied to the liquid crystal is controlled to be parallel with the surface of a substrate by using active elements (hereinafter referred to as a horizontal electric field type), no consideration is given to the characteristic of the light source required to decrease the power consumption of the whole liquid crystal display apparatus. Further, no consideration is given to the configuration of the liquid crystal display apparatus required to suppress color shift in response to the application of a voltage thereto and to prevent a color defect from occurring.
In the establishment of a horizontal electric field, opaque electrodes are provided in a display pixel portion in order to produce an electric field substantially in parallel with the surface of the substrate. As compared with the prior art type of display panel wherein an electric field is applied in a direction substantially vertical to the surface of the substrate by using a transparent electrode, the aperture ratio may be deteriorated and the brightness under a bright state may be reduced. Accordingly, it is necessary to use a high-intensity light source in the horizontal electric field producing type of display panel.
Because the display mode effective for a liquid crystal display apparatus of the horizontal electric field type is a double refraction mode, the transmittance T can be generally expressed by the following equation (1):
                    T        =                  To          ⁢                                          ⁢                      sin            2                    ⁢          2          ⁢                      θ            ·                                          sin                2                            ⁡                              (                                                      nd                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    n                                    λ                                )                                                                        (        1        )            where, To designates a coefficient and is determined mainly by the transmittance of the polarizer used in the liquid crystal panel, θ designates the angle between an effective optical axis in the liquid crystal layer and a transmittance axis for polarized light, d designates the thickness of the liquid crystal layer, Δn designates the anisotropy of the refractive index of the liquid crystal layer, and λ denotes the wavelength of light. Because the transmittance of the liquid crystal display apparatus has essentially a maximum value at a certain wavelength, the liquid crystal display elements are colored. One solution to the above equation is a value which satisfies a condition wherein the peak wavelength becomes equal to the maximum wavelength 555 nm for luminous efficiency under a retardation of 0 order, that is, (πd·Δn/555)=π/2. In this case, the transmittance falls suddenly on the short wavelength side of the peak wavelength, and it decreases gradually on the long wavelength side. Therefore, the liquid crystal display elements are colored yellow. As a result, it is required to use a light source with the color of a cold color family which represents a complementary color to yellow. In other words, it is required to use a light source with a high color-temperature characteristic.
In general, a fluorescent lamp is used as a light source for a liquid crystal display apparatus. Because the luminous efficiency of the fluorescent lamp in a short wavelength region is less than that in a long wavelength region, the brightness may be reduced, and so a large consumption of power is required to obtain a high brightness. Since the normal voltage of the battery must be maintained for a long time, for example, in a note book type personal computer or personal digital assistance, it is required to avoid any increase in the power consumption.
Now, the display operation of a liquid crystal display apparatus of the horizontal electric field type can be obtained in the double refraction mode, and the transmittance T can be generally expressed by the following equation (2):T=T0·sin2 2θ·sin2 [(π·deff·Δn)/λ]  (2)where, T0 designates a coefficient and is determined mainly by the transmittance of the polarizer used in the liquid crystal panel, θ designates the angle between an effective optical axis in the liquid crystal layer and a transmittance axis for polarized light, deff designates the thickness of the liquid crystal layer, Δ denotes the anisotropy of the refractive index of the liquid crystal layer, and λ designates the wavelength of light. Further, the product of deff and Δ is referred to as retardation. Here, the thickness deff of the liquid crystal layer is not the thickness of the whole liquid crystal layer, but the thickness of the liquid crystal layer in which the direction of alignment is changed when a voltage is applied.
In general, the molecules of the liquid crystal in the vicinity of the boundary surface of a liquid crystal layer do not change the alignment direction due to the effect of anchoring at the boundary surface even if a voltage is applied. Accordingly, when the thickness of the whole liquid crystal layer sandwiched between the substrates equals deff, deff<dLC always is maintained between the thickness dLC and deff. It is estimated that the difference between dLC and deff equals about 20 nm to 40 nm.
As clearly seen from the above equation (2), the transmittance of the liquid crystal display panel takes a maximum value at a specific wavelength (peak wavelength). Therefore, the liquid crystal display element is easily colored, in other words, it is easy to be unnecessarily colored.
Generally, the liquid crystal panel is constructed so that the peak wavelength may become equal to the maximum wavelength 555 nm for luminous efficiency, that is, (πd·Δn/555)=π/2. At this time, the liquid crystal display element is colored yellow, because the spectral transmittance falls suddenly on the short wavelength side of the peak wavelength, and it decreases gradually on the long wavelength side.
The extent of coloring extremely changes with the application of a voltage to the liquid crystal. As the magnitude of the voltage value changes from the minimum voltage required for display to the medium tone display voltage and then to the maximum voltage, the color tone is gradually changed. Therefore, the display state of colors is extremely deteriorated.
Because the difference between the thickness of the liquid crystal layers appears as a change in the peak wavelength in the birefringence mode, the local and abnormal thickness of the liquid crystal display layer causes display defects, such as variations in the intensity and/or color tone, which are different from those in its surrounding area.